The electrical properties of a semiconductor can be modified by introducing a controlled amount of impurities (dopants) to the semiconductor crystal, enabling the formation of n-type and p-type semiconductor materials.
Most commonly, doping of semiconductors is achieved by diffusion or ion implantation. In ion implantation, a high energy beam of dopant ions is directed at the substrate to be doped. The ions penetrate the semiconductor crystal causing damage and introducing stress to the lattice. Anneal step is required in order to electrically activate the dopants and relaxation of the stresses. The resulting doping depth achieved by this method is usually microns scale.
Alternatively, doping may be achieved by diffusion. In this process, elevated temperature is used to diffuse dopant atoms from gas or solid phase at the surface (spin-on dopant) into the semiconductor. Diffusion process is usually carried out in conventional furnaces for long periods of time, ranging from tens of minutes to hours. These long time periods are required for the thermal stabilization of the furnace due to the large heat capacity. Long diffusion times typically result in diffusion profiles on the scale of microns. For short time processes, Rapid Thermal Processing (RTP) is used. In RTP, high speed heating is achieved by using, e.g., optical radiation, allowing processes on timescale of seconds.
Doping by thin films is a two steps process. First, a thin film of the desired dopant is formed on the surface of the substrate to be doped. This film serves as a dopant source and anneal step is used to diffuse the dopants into the semiconductor by either conventional anneal or RTP. The thin film can be applied by adsorption from vapor phase [1], atomic layer deposition (ALD) [2], or by spin coating to form a thin solid SiO2 film containing the desired dopants (the so-called “spin-on dopant” method, SOD) [3].
Monolayer Doping (MLD) is a method used for ultra shallow doping of Si [4]. In MLD, intrinsic silicon surfaces are reacted to form single molecular layers (monolayers, MLs) having dopant atoms such as boron or phosphorus. Then, a thin layer of silicon dioxide is evaporated on the substrate as a protective capping layer for the diffusion, and thereafter an activation step follows. The RTP process is used to both diffuse the dopants into the semiconductor crystal and electrically activate them.
Prior to monolayer formation, surface activation is required in order to obtain a uniform monolayer on the entire surface. For example, for Si—C covalent bond formation HF treatment is required in order to remove the native SiO2 layer. Hydroxylation is necessary for binding the precursors to the surface, by hydrogen bonding [5]. This is achieved by removing adsorbed contamination and hydroxylation of surface sites.
MLD has been further used for creating patterned doping on silicon by using MLD together with nano imprint lithography [6] and the resulting doping patterns were mapped by TOF-SIMS imaging. Furthermore, MLD was used for sulfur doping of InP nanopillars [7] for the formation of photovoltaic devices.